Effect of conditions of manufacture on the properties of ultrafine nickel powder

Abstract

The steady expansion of the fields of applications of ultrafine powders (UFPs) necessitates studying and developing their methods of manufacture. In particular, there is a need to identify the factors influencing the particle size of UFPs. Of special interest in this connection is the levitation method [i], which enables UFPs to be produced with very narrow particle size ranges and mean particle sizes varying from 0.01 to 0.5 ~m in a continuous process. The present work was therefore undertaken with the aim of examining, using nickel UFPs as an example, the effect of the key parameters of production of particles by the levitation method on their size. Use of a ferromagnetic material made it possible to study at the same time the effect of the dimensional factor on the magnetic properties of such UFPs. The first results of this investigation have been reported in [2]. As can be seen from the diagrammatic representation shown in Fig. I, evaporation of a metal drop i in the apparatus employed is performed in a suspended state with the aid of a countercurrent generator 2 supplied from a VChG4-10/0.44 high-frequency generator 3. Nickel vapor entrained by an inert gas stream condenses, with the formation of spherical particles (Fig. 2), which are trapped by a filter 4. At the end of the process the particles are removed from the filter by means of a rod 5 into a capsule 6, another rod 7 serving to withdraw a sample of the particles for electron microscopy. During evaporation fresh metal is continually supplied to the drop from a wire of the starting material 8, which passes through a feeding device 9 and a stuffing box I0. The velocity of the cooling inert gas stream v is regulated by means of a valve ii and a rotameter 12. In our work all investigations were carried out at an inert gas pressure close to atmospheric. A study was made of the effect of the rate of particle condensation s, rate of flow of the cooling inert gas, and its atomic weight on the size and magnetic properties of the nickel particles being formed. Magnetic properties were studied using a vibration magnetometer. The rate of condensation for levitation under self-regulating conditions is usuallymade equal to the rate of consumption of the metal wire [i], which can easily be regulated by varying the speed of rotation of the feeding device rollers. In the apparatus employed the rollers were driven by an RD-09 reversing motor through a special reductor. The linear wire feed rate in the system was varied steplessly and in steps in the range 0.03-30 m m/sec (for 0.2-mmdiameter nickel wire the corresponding variation of s was from 0.01 to i0 mg/sec). At constant rates of _fl~ of inert gas through the vaporizer the dependence of the mean nickel UFP particle size d, calculated on the basis of analyses of micrographs, on s was as shown in Fig. 3. The power-function character of the ~ vs s relationship was maintained wit h various inert gases and at various gas flow rates. The slopes of the straight-line plots in the figure indicate a typical d ~ s~/~ relationship.